Blender is applying only part of texture onto plane ? -- No Shading -- Bug?
I think your problem is in view port settings.
As you wrote, your image RGB goes over variety of colors and the corners should give absolute number of the color (as 1.1.0 in case of yellow) This can be picked by color picker in UV editor or texture paint as the texture is shown without any view port shading.
In fact, the view port has some basic shading and light from the very beginning and the colors in 3D view port are always distorted by the environment as the pure color is not reachable in a real world normally.
There is no problem with the texture. If you want the absolute color output you have to set up you render, color or material to give you this output.
If you want it to be seen just in view port then change your shading.